The present invention is in the field of technical ceramics and particularly relates to refractory cordierite ceramics of low thermal expansion and high porosity that exhibit high strengths rendering them particularly suitable for the production of strong, wall-flow filters for the treatment of combustion exhaust gases.
Cordierite ceramic bodies, especially those formed as honeycomb multicellular structures, are utilized in a number of high temperature applications such as catalytic converters, NOx adsorbers, electrically heated catalysts, molten metal filters, regenerator cores, chemical process substrates, catalysts for hydrodesulfurization, hydrocracking, or hydrotreating, and filters such as diesel exhaust particulate filters.
In diesel exhaust filtration, cordierite, being a low-cost material, in combination with offering a low coefficient of thermal expansion (CTE), has been a material of choice. Porous cordierite ceramic filters of the wall-flow type have been utilized for the removal of particles in the exhaust stream from some diesel engines since the early 1980s. A diesel particulate filter (DPF) ideally combines low CTE (for thermal shock resistance), low pressure drop (for engine efficiency), high filtration efficiency (for removal of most particles from the exhaust stream), high strength (to survive handling, canning, and vibration in use), and low cost.
In applications requiring the removal of nitrogen oxides, NOx, from diesel engine exhaust gas, large amounts of catalyst or NOx adsorbers are typically required. In order to minimize increase in pressure drop, high porosity and coarse pore sizes are desired to accommodate the additional NOx catalyst/adsorber. However, both an increase in porosity and larger pore sizes tend to reduce the strength of the ceramic honeycomb.
Further, very low CTE has often been pursued as a means for increasing thermal shock resistance (TSR), and low CTEs in cordierite and some other advanced ceramics have generally been achieved by the presence of microcracking in the ceramic material. Unfortunately, however, the microcracking also serves to further lower the strength of these high-porosity bodies.
Cordierite ceramics offering a combination of high porosity and coarse pore size, e.g. 64-80% porosity with median pore diameters of 10 to 45 μm, have been produced with either very low or very high CTEs. The high CTE ceramics tended to exhibit poor thermal shock performance, while the extensive microcracking associated with the very low CTE ceramics yielded low strength and poor mechanical durability. The latter ceramics are generally of insufficient physical strength as determined by modulus of rupture (MOR) testing for practical use in the mechanically harsh environment of a typical diesel engine exhaust system.
It would thus be an advancement in the art to provide cordierite ceramic bodies that combine both high porosity and coarse median pore diameter for low catalyzed pressure drop, and at the same time intermediate CTEs insuring an improved combination of thermal shock resistance and strength.